Tar sands, which are also referred to as oil sands or bituminous sands, is a common name of geological formations that contain bitumen, an extremely heavy type of crude oil. Tar sands can have a variety of compositions but typically include, in addition to bitumen, water and mineral solids. The mineral solids can include inorganic solids such as coal, sand, and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it may be buried as deep as two thousand feet below the surface overburden. Tar sands deposits are measured in barrels equivalent of oil. It is estimated that the Athabasca tar sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves is estimated to be about 1.3 trillion barrels.
The bitumen content of tar sands varies from approximately 5 wt % to 21 wt %, with a typical content of approximately 12 wt %. Tar sands also include approximately 1 wt % to 10 wt % water. The remainder is mineral matter such as coal, sand, and clay. Bitumen is best described as a thick, sticky form of crude oil that is so heavy and viscous that it will not flow unless heated or diluted with lighter hydrocarbons. At room temperature, the flowability of bitumen is much like cold molasses.
Extracting usable oil from tar sands typically begins by separating the bitumen from the mineral solids. One conventional process includes mixing the ore with hot water to form a bitumen enriched froth. The froth is separated and further processed to isolate the bitumen product. Conventional water based extraction technologies are capable of separating bitumen from higher grade ore but are unable to economically separate bitumen from lower grade ore. Unfortunately, this means that a significant amount of tar sand ore is not capable of being processed to recover the otherwise valuable bitumen.
Another problem with conventional water based extraction technologies is the low overall recovery rate of bitumen. Unfortunately, conventional extraction processes discharge much of the bitumen in the ore with the tailings. Not only does this reduce the efficiency of the extraction process due to lower recoveries, but it also presents potential environmental problems that must be addressed.
The residual bitumen is not the only problem associated with the tailings from conventional processes. The amount and physical characteristics of the tailings also present significant problems. For example, the tailings from conventional processes typically include two components: (a) wet tailings (commonly referred to as “coarse sand”) that contain about 15 wt % water and (b) a stream of a clay/sand mixture (commonly referred to as “fine tailings”) that contains 70 wt % water. In some circumstances, the total amount of the tailings may be more than the amount of the mined ore, which means that the tailings cannot be returned to the mined area. Some of the tailings must be discharged elsewhere. This creates numerous environmental problems associated with the disposal of the tailings. Furthermore, the relatively high water content of the tailings may require the tailings to be stored in a pond instead of stacked or piled thereby further increasing the problems associated with disposal of the tailings.
Fine tailings produced by conventional processes may be especially problematic to dispose. Fine tailings are typically made up of clay, sand, water, and residual bitumen. Due to the water entrained in the clays, fine tailings can have a sludge-like consistency that lasts indefinitely. Byproducts such as fine tailings can be stored in ponds, but these ponds are costly to build and maintain and can be damaging to the local environment, including the local water supply.
Many conventional methods for obtaining bitumen from tar sands also have serious technical limitations. For example, many conventional methods use water, which can cause clays in the tar sands to swell and interfere with processing equipment. In addition, some conventional methods result in the undesirable precipitation of soluble asphaltenes.
One example of a conventional method is described in U.S. Pat. No. 4,046,668 (the '668 patent). The '668 patent discloses the extraction of hydrocarbons from tar sands with a mixture of light naphtha having from 5 to 9 carbon atoms per molecule and methanol. The method disclosed in the '668 patent is limited, in part, because it requires the simultaneous use of two solvents, which increases processing costs.
U.S. Pat. No. 4,347,118 (the '118 patent) discloses a method in which pentane is used to extract bitumen from tar sands. The method disclosed in the '118 patent requires the use of two fluidized bed drying zones. Operation of these fluidized bed drying zones requires a large amount of energy, limiting the efficiency of the overall method. Furthermore, the pentane solvent does not solubilize the asphaltene fraction of the bitumen that is not pentane soluble. Thus, this fraction of the bitumen is discharged with the tailings. For Athabasca type bitumen this can amount to 20 wt % to 40 wt % of the total initial hydrocarbon content of the tar sands.
U.S. Pat. No. 5,143,598 (the '598 patent) discloses a method that includes adding heptane to tar sands to form a bitumen-rich heptane phase and then displacing the bitumen-rich heptane phase with water. This method utilizes steam vaporization and condensation, which are low-efficiency processes. Also, the use of heptane, a non-aromatic solvent, in this method can result in the precipitation of the heptane insoluble asphaltene fraction present in the bitumen phase. The heptane insoluble asphaltene fraction is discharged with the tailings. In addition, using water not only generates large amounts of aqueous waste but also creates oil-water emulsions that are very difficult to breakdown. The use of water can also introduce undesirable impurities into bitumen, such as chlorine, and can result in undesirable swelling of clays in the tar sands. Furthermore, the bitumen recovered by this method typically has a low purity and requires additional processing, such as by centrifugation. This further increases the cost of the overall recovery process.